Your brother needs to find out where the people who buy art hangout online, or even offline. See what websites/blogs they visit and which ones do they like the most then analyse those sites. Check youtube channels, tumblr blogs, facebook groups etc. you need to find similar websites and track/analyse how they get traffic, follow their social profiles and subscribe to their email lists as well.
Also check who's linking to their website online, who shares their content, there are many tools online to do all this stuff and there's also the manual way as well.
But your aim should be to check what other similar successful websites are doing and then replicate. This is a good way to start then you can filter out what's not working and stick to what's bringing you great results.
Build relationships with other website owners in that niche. Search for forums and contact the moderators to see how they can help you help them. Remember, people will usually help you if they're benefiting so always come-up with ways to give before you can ask people for anything, especially when building a long term relationship.
Most websites/blogs are always looking for fresh content and new ideas, so come up with great content (article, video, audio, infographic...) that you can give to high traffic websites in the art niche in exchange for a link to your websites. This is called guest blogging, do some research about it.
Also add value to the marketplace, this will build trust for your website visitors and make your selling process a whole lot easier. A lot of people fail because all they want to do is sell, sell and just keep selling.
Make sure that your websites are entertaining, informative, add visuals (you already have picture, also add some videos and maybe some infographics as well) and the content needs to be worth sharing (engagement is really important, especially shares or retweets/repins/reblogs etc.).
So don't focus on making money only, also add value to the marketplace and you'll get more traffic.
Another great marketing technique is to feature some popular names in the art niche, maybe do some interviews as well. This will be valuable content for most people. If you're struggling to come up with content then check popular magazines, you'll find tons of topics there even by just looking at the cover.
Hope that helps.
